There are many net classified sites where you can place your free classified ads. Some of the finest ones are Craigslist, Kijiji (owned by eBay), BackPage, and Hoobly but you can see plenty more if you try a search on any search engine. Sites vary in what the free membership includes and you can always upgrade to a moderate monthly charge if you feel that it's a necessity to get the features you need.
In any case, post as many classified ads as the site allows. You must change the announcements up to use different keywords and market areas areas that your business tends to. If you can go in every day and just change one letter or make a tiny adjustment to your ad, it'll remain close to the top as the site will view it as something new. You'll likely need to go through a little bit of trial-error with many alternative adverts before you start to see what it truly working and what isn't.
Another reason the net and advertising are so compatible is perhaps because your ad will have a hyperlink to your website. So your audience just clicks on the link and they're immediately at whatever it is that you need to offer. Clearly, offline advertising doesn't offer this straightforward transition. Even if you capture your audience's interest, they may simply forget about your internet site when it comes time for them to go online.